1. Field of the Invention
This invention relates to a magnetoresistance device, and more particularly to a magnetoresistance device that enables scale-down.
2. Related Background Art
Magnetoresistance devices are used in reproducing heads of hard-disk drives at present, and have become indispensable to hard disks having a high recording density. Such devices are also used as sensors and besides are studied on their application to solid-state memory devices.
Anisotropic magnetoresistance devices used in the reproducing heads are basically ferromagnetic films having in-plane magnetic anisotropy, and have a circuit that flows electric currents in the in-plane direction of the ferromagnetic film and a circuit that detects changes in resistance of the ferromagnetic film.
FIG. 1 illustrates the principle of anisotropic magnetoresistance effect. An electric current I flows in the easy-magnetization direction. A ferromagnetic film 1 is provided so that a magnetic field H is applied in the in-plane hard-magnetization direction. A resistivity xcfx81 when the angle at which magnetization M and electric current falls in xcex8 can be represented by an equation:
xcfx81=xcfx81∥xc2x7cos2xcex8+xcfx81xe2x8axa5xc2x7sin2xcex8
wherein xcfx81∥ is a resistivity in the case where the magnetization direction is parallel to the direction of electric current, and xcfx81xe2x8axa5 is a resistivity in the case where the magnetization direction is perpendicular to the direction of electric current.
As can be seen from this equation, the resistance of the ferromagnetic film 1 depends on an angle which the direction of the electric current flowing therethrough and the direction of the magnetization make. In the reproducing heads, the magnetization direction of the ferromagnetic film changes depending on floating magnetic fields from the hard-disk, and an amount of its change is detected as an amount of change in resistance.
FIG. 2 shows magnetization directions and a change in resistivity of a magnetoresistance device used as a memory. The case where the magnetization is rightward is regarded as xe2x80x9c0xe2x80x9d, and the case where the magnetization is leftward, as xe2x80x9c1xe2x80x9d, where a rightward magnetic field greater than the coercivity of the ferromagnetic film is applied at the time of detection. In this case, the magnetization direction of a ferromagnetic film where xe2x80x9c0xe2x80x9d has been recorded does not change, but the magnetization direction of a ferromagnetic film where xe2x80x9c1xe2x80x9d has been recorded reverses. When magnetization reverses, the magnetization inclines to the direction of electric currents, hence the resistivity changes as stated above. Thus, xe2x80x9c0xe2x80x9d is detected when there in no change in resistivity, and xe2x80x9c1xe2x80x9d is detected when there is a change.
On the basis of such a principle as mentioned above, in the recording or detection in conventional magnetoresistance effect type memory devices, conductor wires 31 and 32 are provided at the top and bottom of a ferromagnetic film 40 as shown in FIG. 3, and electric currents are flowed through them, whereby a magnetic field is applied to the ferromagnetic film in-plane.
The magnitudes of electric currents to be flowed through the top and bottom conductor wires are so determined that the magnetization of ferromagnetic film 40 positioned at the part where the conductor wires cross does not reverse when only the magnetic field generated from one-side conductor wire is present but the magnetization reverses when electric currents are simultaneously flowed through the both wires. The direction of a magnetic field to be applied at the time of recording depends on the direction of an electric current flowed through the conductor wire provided in the direction perpendicular to the magnetic anisotropy of ferromagnetic film 40.
The ferromagnetic films used as magnetoresistance devices are ferromagnetic materials formed of Ni, Fe or Co or an alloy of any of these and have an in-plane magnetic anisotropy. The induction of such an in-plane magnetic anisotropy is commonly accomplished by applying a magnetic field in the direction where a ferromagnetic film is made to have a magnetic anisotropy during its formation.
Now, when data having a vast volume as exemplified by recorded voices or sounds and recorded images are handled in, e.g., mobile information appliances, the data are recorded in disks or tapes. Such information appliances, however, require drives and hence require power sources having a large capacity. Since such drives (e.g., a motor) and large-capacity power sources are set in, it has been difficult to make the appliances light-weight.
In addition, in the case of mobile information appliances whose recording mediums are solid-state memories, any solid-state memories made to have a sufficiently high recording density have not been materialized, where only data having a small volume can be handled. As a reason therefor, this is due to the problem that, although the scale-down of ferromagnetic films is required in order to materialize magnetoresistance memories having a high recording density, the demagnetizing field increases with a decrease in size of the ferromagnetic film in its easy-magnetization axis direction and hence the magnetization becomes unstable, resulting in a poor recording storage performance of memory devices.
Accordingly, an attempt is proposed in which the magnetic anisotropy of a ferromagnetic film of a magnetoresistance memory is directed to the direction perpendicular to its film plane (film-plane normal direction) to make its recording density higher.
However, materializing magnetoresistance memories of this type has involved the following problems.
Taking account of their setting in mobile information terminal appliances, it is desirable for them to be operable at a low electric current. Accordingly, materials having a small coercivity or a small magnetization-saturating magnetic field are desirable as ferromagnetic film materials. Such materials may include rare-earth/transition metal alloys, in particular, materials making use of gadolinium as the rare-earth metal.
However, in rare-earth/transition metal alloy films, the mean free path of electrons is as very short as several angstroms. Hence, in magnetoresistance devices of a spin scattering type making use of such films, it is difficult to obtain a high magnetoresistance ratio.
Spin tunnel type magnetoresistance devices making use of a rare-earth/transition metal alloy also show a tendency to decrease in the rate of spin polarization when rare-earth metal atoms are present at the interface between the film and a non-magnetic layer.
Moreover, since rare-earth metals tend to be oxidized, there is a problem that, when oxides are formed in contact therewith, the rare-earth metals are oxidized to cause a lowering of magnetic characteristics or the magnetoresistance effect.
Taking account of the above problems, an object of the present invention is to provide a magnetoresistance device having a relatively high magnetoresistance ratio, also enabling scale-down and low-current driving, and may cause less deterioration of recording storage performance.
To achieve the above object, the present invention provides a magnetoresistance device comprising;
a multi-layer film having a first magnetic layer, a second magnetic layer, a non-magnetic layer, a third magnetic layer and a fourth magnetic layer which are superposed in this order;
wherein the first magnetic layer stands exchange-coupled with the second magnetic layer, and the third magnetic layer exchange-coupled with the fourth magnetic layer, and both the first magnetic layer and the fourth magnetic layer have a magnetic anisotropy in a normal direction of a plane of the film.
Details and advantages of the device will become apparent from the description of embodiments given later.